The present invention relates to a communication network control system and, more particularly, to a control system for a local area network (LAN).
Communication network control systems know in the art include (a) a carrier sense multiple access (CSMA) baseband LAN, (b) a broadband LAN, (c) time division multiple access (TDMA) baseband LAN and digital PBX, (d) a system disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 57-104339, (e) a system disclosed in Jpanese Laid-Open Patent Pubication (Kokai) No. 58-139543, (f) a system disclosed in Japanese Patent Application No. 60-170428, (g) a system disclosed in Japanese Patent Application No. 60-170429, and (h) a system disclosed in Japanese Patent Application No. 60-170427.
The system (a) proves effective when packets are short and occur in bursts like data information and text information. However, a problem with the system (a) is that when packets are potentially infinite in length and are generated continuously and real time processings of the packets are required, such as in multi-media communications, signals conflict very frequently to limit the throughput attainable. The words "multi-media communications" as used in the art of LANs covers not only the traditional data and text communications but also the interchange of image information, audio information and video information. The system (b) is rather short in capacity when applied to multi-media communications and, in addition, it is not fully acceptable in the aspect of cost performance and expansibility. As regards the system (c), although it has superior applicability to multi-media communications to any of the others, it also has problems left unsolved with regard to costs and expansibility; especially the costs are prohibitive when the system is applied to multi-media communications.
The systems (d) and (e), on which the present invention is based, are the most feasible with respect to multi-media communications. Nevertheless, because both of the systems (d) and (e) are operated with a first-come-first-served logic and on a multiple-input one-output basis, i.e., a node which has received call requests from a plurality of terminals accepts only the first request to transfer a call from a single terminal which has originated the request, connection control means which is installed in the node cannot allow a plurality of different communications to be effected in parallel.
The system (f) is a solution to the problems particular to the systems (d) and (e) as stated above. Specifically, in accordance with the system (f), a single node is capable of dealing with a plurality of communications at the same time, that is, a path with a particular pattern is fixed to prevent one communication from interfering with others. While such a scheme successfully promotes effective use of links and, thereby, interchange of massive data which is desirable for a large-scale LAN having a plurality of nodes, the system (f) is not applicable to a small-scale LAN in which the number of terminals is small.
Meanwhile, it is usually necessary for a call packet to be propagated through a plurality of nodes before reaching a terminal called. A preamble which heads a packet is cut little by little at each of the nodes to bring about a propagation delay, which in turn correspondingly increases the period of time during which a path is fixed to thereby increase the probability of conflict of the packet with others. The system (g) is elaborated to elminate the propagation delay so that the period of time during which a path is fixed may be shortened. Further, the system (h) sets up semi-duplex communication to allow a single node to perform communications over a plurality of channels, while the system (g) sets up semi-duplex communication which is directed to the increase of communication capacity. However, a drawback with such systems on which the present invention is based is that they are incapable of detecting conflicts or collisions of packets. Although the probability and the influence of collisions are less in such systems than in the others, it is desirable that even the faults with least frequency be eliminated when it comes to highly reliable systems.
As regards the reliability of communication systems, countermeasures have been proposed against (a) disconnection of a link, (b) down of a terminal or that of a node, and (c) collision, as will be described hereinafter.
To begin with, in a carrier sense multiple access with collision detection (CSMA/CD) coaxial cable baseband LAN as typified by the Ethernet system (Xerox), upon disconnection of a link a communication with a station on the other side of the point of disconnection is disable, and event a communication with a station on this side of the same point cannot be effected normally since a signal is reflected at that point. Concerning a failure of a terminal (tap, transmitter/receiver, NTU, etc.), it would not affect communications so long as only the functions of the terminal were disabled. However, if the failure was of the kind transmitting unexpected signals, it would destruct signals of all the communications. Further, this system is apt to cause collision frequently since it is based on a contention-for-single-bus principle; every time a collision occurs, information has to be retransmitted at the sacrifice of traffic.
In a TDMA optical fiber loop LAN, all the communications are shut off when a link is disconnected. To avoid this, such a LAN is usually provided a double-link configuration so that a communication may be returned over a new loop before it reaches a point of disconnection. Also, all the communications are disabled when a node which does not join in a communication under way has failed (functions disabled). Usually, such an occurence is handled with by doubling the link so that a communication may be returned over a new loop before it reaches the node in failure. Furthermore, when a node which is not taking part in a communication has failed and sent an unexpected signal, signals of all the communications are destroyed. Various controls adapted to settle the above sitations increase the total cost of the network. While a single supervisor node is installed in the network for the control of the entire network, it brings about another problem that if the supervising node fails, all the communcations are shut off. As regards collisions, because the capacity is substantially distributed to the nodes, there is no chance of a collision to occur although the capacity is limited.
As stated above, in the prior art LAN systems, the contention for a common bus type scheme cannot avoid frequent collisions and, due to retransmission which is required at each time of collision, lowers the traffic. In addition, in the case of a system which cannot detect a collision, a fault would occur to down the entire system.